1. Field of the Invention
The present invention relates to a novel lamination of materials which provides a magnetically stable shield for an MR head and more particularly to a shield which returns to a common state so that after perturbation by a magnetic field, magnetostatic coupling between the shield and an MR sensor remains stable.
2. Description of the Related Art
An MR head includes an MR sensor which is sandwiched between first and second gap layers which are, in turn, sandwiched between first and second shield layers. In a disk drive the MR head is mounted on a slider which is connected to a suspension arm, the suspension arm urging the slider toward a magnetic storage disk. When the disk is rotated the slider flies above the surface of the disk on a cushion of air which is generated by the rotating disk. The MR head then plays back recorded magnetic signals (bits) which are arranged in circular tracks on the disk. In high density disk drives bits are closely spaced linearly about each circular track. In order for the MR head to playback the closely spaced bits the MR head has to have high resolution. This is accomplished by close spacing between the first and second shield layers, caused by thin first and second gap layers, so that the MR sensor is magnetically shielded from upstream and downstream bits with respect to the bit being read.
The MR sensor is a small stripe of conductive ferromagnetic material, such as Permalloy (NiFe), which changes resistance in response to a magnetic field such as magnetic flux incursions (bits) from a magnetic storage disk. The MR sensor receives a sense current and is connected to signal processing circuitry. When the sense current is transmitted through the MR sensor the processing circuitry detects changes in potential which are caused by changes in resistance of the MR sensor. The potential changes correspond to signals received by the MR sensor. The response curve (input vs. output) of an MR sensor has linear and non-linear portions. It is important that the MR sensor respond along its linear portion so that the MR sensor has a linear response. This is accomplished by magnetically biasing the MR sensor at a biasing point on the response curve which is in the linear response portion of the response curve.
An MR head is typically combined with an inductive write head to form a piggyback MR head or a merged MR head. In either head the write head includes first and second pole pieces which have a gap at a head surface and are magnetically connected at a back gap. The difference between a piggyback MR head and a merged MR head is that the merged MR head employs the second shield layer of the read head as the first pole piece of the write head. A conductive coil induces magnetic flux into the pole pieces, the flux fringing across the gap and recording signals on a rotating disk. The write signals written by the write head are large magnetic fields compared to the read signals shielded by the first and second shield layers. Thus, during the write operation a large magnetic field is applied to one or more of the shield layers causing a dramatic rotation of the magnetic moment of the shield layer.
Unfortunately, prior art shield layers are not stable when subjected to a large field. Sendust (FeSiAl), which is a typical shield material, is almost isotropic with an intrinsic uniaxial magnetic anisotropy of only about 1 Oe. This means that magnetic domains within the Sendust material are not well configured with respect to the MR sensor. The walls of the domains are random and when the shield is subjected to a large applied field, such as the write head field, the domains walls move and then return to a different random arrangement. Accordingly, there is a change in the stray magnetic field produced by the shield layer. Because of a magnetostatic coupling between the MR sensor and the shield layers the change in the stray magnetic field of the shield changes the bias point of the MR sensor which, in turn, changes the response of the MR sensor to signals from the rotating disk. The result is noise during the read operation. To make matters worse Sendust typically exhibits stress induced anisotropy due to its magnetostriction. This stress, which may be tensile or compressive, is developed by thin film construction and/or by lapping of an air bearing surface (ABS) at the flying surface of the head. Since the stress induced anisotropy can easily exceed the intrinsic anisotropy the stress induced anisotropy will control domain configuration. It could re-orient the domain structure in undesired direction depending on the magnitude of stress and magnetostriction.
In order for domain walls to be well configured they should be parallel to the easy axis of the MR sensor except for a small area of closure domains which are at each end of the shield layer. Further, the domain walls should always return to the same wall configuration after perturbation by a large applied field so that the magnetostatic coupling between the shield and the MR sensor remains a constant. This can be accomplished if the Sendust material is provided with sufficient intrinsic uniaxial anisotropy. Another material which has been considered for shields is iron nitride (FEN).